-THE ELEVENTH CHESAPEAKE SAILING YACHT SYMPOSIUM

Stars and Stripes Design Program for the 1992 America's Cup Chris Todter, Team Dennis Conner, San Diego, California, USA David Pedrick, Team Dennis Conner, San Diego, California, USA Alberto Calderon, Team Dennis Conner, San Diego, California, USA Bruce Nelson, Team Dennis Conner, San Diego, California, USA Frank Debord, Team Dennis Conner, San Diego, California, USA Dave Dillon, Team Dennis Conner, San Diego, California, USA

ABSTRACT

The Team Dennis Conner (TDC) design program for the 1992 America's Cup is presented in an overview form. The team members are listed. The spectrum of design tools available are discussed, highlighting the usefulness and emphasis of each. The design tradeoffs will be presented in general form, including a discussion of the monoplane/multiplane appendage tradeoffs. The importance of the structural design aspects and methods will be presented. An appreciation of the full size performance feedback to the design will be covered.

NOMENCLATURE

b= q= e=

induced drag force for monoplane induced drag force for biplane aircraft weight (which, for the yacht becomes the force perpendicular to the keel's hydrodynamic axis and to the remote apparent water velocity geometric span remote dynamic water pressure efficiency factor Dib=induced drag force for biplane Lift on wing of biplane with span 1 Lift on wing of biplane with span 2 geometric span of wing 1 geometric span of wing 2 remote dynamic water pressure a complex function of span ratio and gap to span ratio, independent of stagger

INTRODUCTION

Following is a discussion of the overall design program of the Team Dennis Conner America's Cup effort for 1992. The team members are listed, scheduling strategy is discussed, the design tools used are described, and the methodology of the design process is shown to the extent possible within the limitations of confidentiality restrictions, and within the scope of this paper.

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TEAM MEMBERS

The Design Team consisted of the following members:

PRINCIPAL DESIGNERS David Pedrick Pedrick Yacht Designs

Alberto Calderon Advanced Aeromechanisms Corp

Bruce Nelson Nelson-Marek Yacht Design

TECHNICAL COORDINATOR Chris Todter

TEAM DENNIS CONNER OPERATIONS MANAGER Bill Trenkle

TECHNICAL ANALYST Frank Debord Scientific Marine Services

ASSOCIATE DESIGNERS Matt Brown Advanced Aeromechanisms Corp

T.J. Perrotti Pedrick Yacht Designs

Scott Vogel Nelson-Marek Yacht Design

RESEARCH ASSOCIATES Bill Burns Advance Aeromechanisms corp

Scott Ferguson Pedrick Yacht Designs

CONSULTANTS (in alphabetical order)

Boeing Corporation Ed Tinoco, Paul Bogataj, Winfreid Feifel, Bill Herling, Arvel Gentry

General Motors Technical staffs Group

Newport News Shipbuilding Steve Slaughter, Paul Miller, Dave Dillon

Ove Arup Peter Heppel, Patrick Dallard

Rasmussen and Associates Willem Kernkamp

Science Applications International Corporation Eric Schlagater, Carl Scragg, Don Wyatt, John Kuhn

South Bay Simulations Bruce Rosen, Joe Laiosa

CONTEXT FOR THE DESIGN PROGRAM

A new class of yacht was used for the 1992 America's Cup. This new International America's Cup Class was the result of the political and TV pressure to produce a modern exciting match racing boat for San Diego conditions. Several of the Design Team members were involved in the class formulation which produced a deceptively simple Rule formula allowing significant latitude of design trade­offs between sail area, displacement, and characteristic length.

As with any new rule and the intense concentration of talented designers trying to beat the rule, there were numerous interpretations required to keep the event fair and yet not stifle creative progress. This process was skillfully handled by the Technical Director and measurers of the International America's Cup Class (IACC). Generally, the Rule turned out to be very good, with a wide variety of hull forms and parameters present in the 1992 America's Cup and some of the closest racing the America's Cup has ever had.

The reason for this introduction to the new class is to make the point that all the syndicates started with virtually a clean sheet of i;:.aper in terms of design. We were faced with finding the optimum hull/appendage/sail plan combination quickly, with no prior database of design knowledge for this class. This prospect led us to believe that the team who developed the best tools, and learned the fastest would have the best chances of success. In simple terms, given other factors such as financial resources being equal(which of course they are not), the team with the best Velocity Prediction Program (VPP) could win the race. This is a gross simplification, but its basis is true.

STRATEGY

The overall design program was developed around what was initially to be a 3 boat program. A lot of effort went into understanding the strategy related to when to build the boats including, of course, what levels of R & D and anticipated gain increments would be achievable at various points along the way.

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This may seem like a waste of time and effort but our group felt that to compete with the richer syndicates, even with our modest 3 boat program, we had to make every test and every move count. The strategy became more critical and required even more planning as it became apparent that financial constraints would limit us to fewer boats.

The trade offs considered included the desire to have a two boat program for testing certain design aspects such as tandem appendages, or rig proportions at full scale. This thinking was opposed by the desire to put as much R & D into what might be the final design (the second boat), thus waiting to build, which would leave us without a full scale testing program. It was decided to wait as long as possible with the second boat so we could put as much design effort into it as possible. This meant that we had to test multiple appendages in other ways (described later) and study the rig proportion trade-offs by observing our competitors.

Our design program was faced with another setback later in the year after putting an intense effort into what was to be the final hull design, when the "drop-dead" day passed without our syndicate being able to commence building. We had determined the exact timing of the build and outfit cycle for the second construction and had targeted the latest possible day for having the design complete enough for the construction to start. As the date passed, we had to resign ourselves to the fact that modification, possibly including multiple appendages, and refinement were our only avenues of improvement left.

We had, of course, an appendage program underway, both looking at biplane appendages as well as more conventional bulb/keel/rudder improvements, but the emphasis on the appendage aspect was heightened at this point.

DESIGN TOOLS

The tools available to the Design Team, and to some extent any yacht designer who can afford to use them, cover a wide range of technologies, costs, level of accuracy, and confidence. some tools are best suited to hull parametric evaluation, others to detailed appendage design. The selection of the right tool for the particular job and more importantly the interpretation of the results is the key to successful modern yacht design.

Each of the design tools available and used by the Design Team is listed below, with a discussion of their advantages and disadvantages and their applicability.

The towing tank is the classical and ,still the most reliable and relied on design tool, although one of the most expensive.

Methodology. Test procedures and data

analysis techniques were, for the most part,

based on those developed by the Partnership

for America's Cup Technology (PACT) in the

Spring and Summer of 1990. A great deal of

effort was spent on quality control during

testing. Relevant code predictions and

previous tank results were always used during

testing to provide real-time assessment of

results. Data was expanded using the PACT

appendage stripping technique and expanded data

was faired by fitting splines through forces or

moments versus speed, yaw and heel. An example

of raw and faired data is shown in Figure 1.

During data expansion, variable wetted length

and wetted area were used to more accurately

determine wavemaking drag. Although this does

not significantly affect total drag

predictions, it does provide more accurate

wavemaking drag values for comparison with

Computational Fluid Dynamics (CFO) results.

DRAG

FIG 1. TANK DATA

Thanks to sponsorship by Cadillac, Team Dennis

Conner was fortunate to have access to the GM

Technical Center Aerodynamics Laboratory. The

size of this wind tunnel permitted tests with

1:3.5 scale models. This resulted in some

model fabrication cost savings since models

built for the tank could be used in the tunnel

as well, and it permitted direct comparison of

tank and tunnel data at model scale.

Methodology. Tests were completed using

an imaged canoe body mounted with the

waterplane vertical in the tunnel. This

arrangement is illustrated in Figure 2. The

model includes actuators to control yaw angle,

rudder angle and tab angle from the control

room. In addition, it permitted attachment of

image appendages and provided for variable keel

location. Tunnel speeds permitted testing to

keel Reynolds Numbers up to 1.3 x 106.

Typically configurations were tested at the

towing tank Reynold s Number and this maximum

Reynolds Number. Certain configurations were

tested at additional Reynold s Numbers to

provide data for extrapolation.

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FIG. 2 WIND TUNNEL

SPLASH is a linear free surface panel method

potential flow program developed by South Bay

Simulations. It has been under a continual

development process for several years,

producing accurate wave drag, lift and induced

drag estimates for various heel, and yaw

settings.

While the absolute accuracy of the code is very

good, the best use of this type of code is in

doing comparative or derivative studies. This

means that the code is "anchored" to some known

data such as a tank test, then relatively small

geometric variations of the hull or appendage

are run producing differences from the

baseline.

This is a large complex code requiring

experience in the use of similar codes or

preferably the use of SPLASH. The duplication

of a set of tank test results takes several

hours of CRAY computing time and several days

of job submission and interpretation, but when

used properly is a very powerful tool.

Non-Free Surf ace CFO Codes

Into this class of computer flow simulations

fall codes such as the proprietary potential

flow code used by Boeing for wing and body

design, and Boeing's boundary layer codes which

were applied in the design of TDC keels and

bulbs. The same comments made for SPLASH apply

here, the practitioner must be skilled and the

codes require significant computer resources to

run. The results, when properly interpreted,

are excellent.

VPP/RMP

The VPP is the glue that ties all the analysis

and test data together, combining the hydrodynamics of the hull and appendages with the aerodynamics of the sails and rigging.

In the simple case, the VPP takes a set of tank data extrapolated to full size lift, drag, and roll moment, and finds the equilibrium boat speed and wind angle for optimum upwind, downwind, and reaching performance. The weight distribution and rig geometries are additional inputs to the VPP, which produces a table of speeds and times for the various legs of the AC course for selected windspeeds.

The VPP can take hull and appendage data from tank tests, wind tunnel tests, from SPLASH runs, other CFD {any rational or arbitrary alteration of the lift and drag multidimensional surfaces is possible) and produce the estimates for performance.

The VPP can also produce its own internal estimates of hull and appendage performance to use in the equilibrium prediction process. This is called the LPP (Lines Processing Program) and is based on some simplified formulas for wave drag and appendage effective span (originally from the IMS formulation but modified to better predict IACC yacht performance). This process is understandably less accurate than inputting tank data but is still useful for analyzing small variations around a known baseline.

It should be noted that these predictions are only as good as the input data including the assumptions made for the sail forces. This is the reason that full size sailing performance feedback to the VPP is important.

The RMP {Race Model Program) is the tool that takes the performance predictions for various design candidates and tests them against each other (in the computer) using the wind speed data collected over the past ten years from the race venue. The boats are run against each other using the wind as recorded at the appropriate time of day, with realistic match racing ahead and behind penalties. A win/loss percentage is developed for each boat in this manner and thereby the best candidate can be determined.

sea Keeping Codes

It is fundamental that moving a yacht through the water takes energy, and the best performance comes through maximum extraction of energy from the sails and the minimum dissipation of energy by the rest of the yacht in attaining its speed. Punching into waves absorbs energy, which, for a given sail force, reduces the amount of energy available to sustain speed. Pitch and heave motions send additional radiating wave formations away from the hull. Other transient effects of hull, keel, rudder, and sail fluid dynamics accumulate losses compared to more ideal steady-state flow.

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TDC's oceanographic research showed that the sea state off Point Loma is frequently more severe than what is normally associated with a given wind velocity. Therefore, good rough water performance was a factor in the design teams' hurried analysis of Stars & Stripes and then became a priority area of study for the development of the final design.

Seakeeping codes are relatively large computer programs used to predict the motions of and forces acting on vessels and structures caused by ocean waves. These codes have been under development for more than 30 years. The available codes have to date been of limited use to yacht design, however, for several reasons.

The first major reason is that the priorities of ship design are much different than yacht design. A racing yacht design has speed as its main goal, while ship design is more concerned with motions, particularly rolling and slamming. The ship emphasis on motions rather than added resistance, which is required for yacht design, means that relatively little research on added resistance has be undertaken.

The second explanation for the lack of yacht seakeeping codes is that yacht shapes are substantially different than motor yachts or ships, which are essentially "wall-sided" well above and below the waterline throughout most of their length. Yachts have considerable flare in their transverse sections, changing shape dramatically over a relatively small vertical range near their waterline. They nearly always have shallow-sloped after overhangs, and bow shapes vary greatly from the more plumb-profiled, finer waterlined "destroyer" type to the shallow-sloped, blunter-waterlined "spoon" or "meter" type. Furthermore, the heel angle of a sailing yacht is held nearly constant by the sail forces, while rolling ships average upright with large excursions in either direction. Consequently, the assumption of "wall -sidedness", which has simplified some of the codes to the extent of making them solvable (in some cases, they are not solvable without this assumption), has worked well for ships but renders most of the codes unsuitable for yachts.

Given the general difficulty in predicting added resistance, the sailing yacht's flared shape, and asymmetry introduced by heel angle, the previously existing seakeeping codes had very limited application in Stars & Stripes design. PACT's research evaluated several ship codes, and helped identify two promising candidates for yacht design, the Lin-Reed strip theory code, and SWAN, a JD linear panel code attributed to Paul Sclavounos of MIT.

TDC went on to explore the Lin-Reed code.

While it was not particularly good at

quantitative evaluations of different hull

shapes, it proved very useful in predicting

differences in added resistance due to changes

in radius of gyration (distribution of mass

away from the center of gravity), position of

the vertical center of gravity, and in

correcting model test data for the towing

position.

The SWAN code, a more sophisticated and

fundamentally grounded seakeeping code, and

Paul Sclavounos' ray theory bow shaping code

(which consideru the forces caused by the

encountering wave's impact on the 2D bow shape)

were both considered promising and the TDC team

was anxious to try both but they were

considered too developmental at the critical

design time. The SWAN code has since proven to

be an accurate and useful code in subsequent

PACT research.

The limitations of seakeeping codes as of

March, 1991, led to a conunitment by TDC to an

ambitions series of rough-water model tests

(described later). Sufficient testing was

performed to develop generalized, parametric

relationships for added resistance of IACC

yachts from experimental data, with guidance

and refinements from the Lin-Reed seakeeping

code and theory. This ability to predict added

resistance of arbitrary IACC candidate yachts

was added to the VPP/Race Model evaluations in

the last few months before the freeze date of

our final design. Since the final design was

not built, the rough-water capability developed

was used to assist in on-going refinements to

Stars & Stripes.

Structural Codes

Four major groups worked with the principal

designers during the structural design and

analysis phase. Ove Arup performed a

preliminary finite element analysis (FEA) of

the initial structural configuration. Newport

News Shipbuilding (NNS) was involved in the

hull structure and keel detailed design and

analysis, and General Motors did research for

the second boat design. In addition, Rasmussen

and Associates performed hydroelastic analyses

on several of the appendage iterations and

configurations.

In the detailed structural design and analysis

stages, two principal types of structural codes

were used- a laminate analysis program and

finite element analysis programs. The laminate

analysis program was used to compare initial

laminate designs based on stiffness and

strength in the major directions. once this

process was complete, the laminates were input

into the finite element models and evaluated

against a set of applied loads.

The laminate analysis program used by NNS was

GENLAM, a public domain program available

through Think Composites of Stanford

University. GENLAM is based on the laminated

plate theory for analyzing composite plate

structures and will run on almost any personal

computer (PC). GENLAM can calculate the

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stiffnesses and strengths of asymmetric hybrid

laminates and will predict factors of safety

for a set of applied loads based on the TSAI-WU

failure criterion. Since it was possible to

analyze many laminates quickly and dismiss

specific combinations of materials and ply

formats early, much time was saved in the

finite element portion of the analysis.

A finite element program works with a user­

generated mathematical model of the geometry

and material properties of a structure to

perform an analysis that can determine the

deflections and internal stresses in the

structure for a set of applied loads. The four

major commercial finite element codes used in

the analysis of Stars & Stripes were MSC/PAL2,

COSMOS/M, I-DEAS, and MSC/NASTRAN. The

preliminary analysis done by ove Arup was

performed using I-DEAS, Sparcr:aft used an in­

house code, Newport News Shipbuilding worked

with MSC/PAL2 and COSMOS/M and General Motors'

work was with NASTRAN. MSC/PAL2 and COSMOS/M

are both PC-based programs, while I-DEAS and

NASTRAN are most widely used in mainframe or

workstation environments.

The major portion of the finite element work

was accomplished using COSMOSIM. This program

can analyze models with up to 15,000 nodes or

60,000 degrees of freedom while running on a

PC. COSMOS/M allows for input: of geometric

data via a generic graphic format file (such as

.DXF), output by programs such as AUTOCAD.

This made for easier model building by allowing

the direct input of lines from the designers.

COSMOS/M can also output model files for

NASTRAN analysis, which aided in transferring

model changes between NNS and General Motors.

scale Sailing

Scaled testing relies on the principle that if

all linear dimensions are accurately scaled,

other than second order effects caused by the

vertical wind speed gradient and foil and sail

Reynolds number degradation, the performance of

the scaled yacht can be extrapolated to full

scale. This method has some benefits over full

scale, primarily lower cost, but still suffers

from the inherent difficulties of boat-on-boat

testing.

TDC used this method for some intermediate

level testing of the tandem keel configuration

as discussed below.

Full Size Data Analysis

The analysis of full size sailing data is

extremely important to the design process since

it provides the feedback on how well the

performance predictions are borne out in real

life. A proprietary data analysis package was

developed to take the logged data such as boat

speed, wind speed, wind angle, and heel and

apply dynamic adaptive filtering algorithms to

the data to reduce the noise. This smoothed

data was then assembled statistically to

produce sailing targets for real time

optimization. These targets also served to

quantify the performance envelope for

comparison with the VPP predictions.

FIG 3. TYPICAL POLAR DATA PLOT

DESIGN OF STARS & STRIPES (USA 11)

Introduction

The first boat was commenced as early in the program as possible, to give us a boat for the May, 1991 World Championships. The basic parameter set for the first boat was defined in just two days, wi~hin three weeks of first assembling the design team. The concept for the first boat was for it to be a test platform and it was to be our best effort at an optimized design for San Diego conditions while consciously avoiding parametric extremes which would have limited our flotation and tuning flexibility.

The keel/mast step/chainplate structure was designed to allow radical movements or alternate configurations of appendages and as such was an overkill for a standard fixed position rig and keel, a penalty she still carries.

The deck layout and in fact deck construction had been done earlier (ahead of the boat) to allow us as much hull design time as possible.

The detailed internal structural design and skin layup was developed over the course of the ensuing construction with drawing being delivered just before construction.

Parameters

The design team reviewed all the data available at that time which included the limited TDC tank testing described below, the PACT series of tank tests, some pictures of the French and Italian boats (the first two IACC boats built), some wave drag CFO runs by South Bay Simulations (on PACT hulls), and numerous VPP runs including a rough water model based on the analytical approach developed during the Stars & Stripes 1987 design program. A basic parameter set of length, displacement, sail area, and beam were determined.

The principal designers then developed several iterations of hull lines plans until all offices were satisfied and the plug for the hull was started.

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There were numerous other basic decisions which had to accompany, or closely follow the hull lines plan development, such as:

a. main sail area versus foretriangle area b. estimates of the structural requirements

and weight c. size of fin and rudder d. bulb shape e. mast and rigging stiffness

Fortunately, the design group had been engaged independently in this research for the previous year and needed only refine the calculations based on the final range of length, weight,

sail area.

TDC Towing Tank Program

In addition to relying heavily on early parametric tank test results provided by PACT, Team Dennis Conner completed model tests at four different towing tanks. Late in the summer of 1990 several models were tested at 1:8 scale at the Davidson Laboratory, exploring choices in hull size, proportions and bow shape. These tests were used to extend the PACT parametric series and results were tied to

the 1:3.5 PACT scale tests by duplicating one PACT scale model.

In November, 1990, after commencing construction, a series of 1:3.5 scale tests were initiated at Arctec Offshore Corporation with a model of USA 11. For the following 14 months the team used this facility as the primary source of calm water tank tests. Early

in the development of the tandem appendage configuration, exploratory tests were completed at small scale at a university tank. In addition, the team completed proprietary seakeeping tests at the University of Michigan.

Table 1 summarizes the 1:3.5 scale tests completed for development of a second hull and optimization of USA 11. These tests were closely related to the wind tunnel test program and the SPLASH CFO work described in the following sections.

The first series of these tests was used to characterize USA 11, establish sensitivity to small changes in displacement and complete some appendage stripping exercises. Next a set of appendage tests was completed to provide guidance in planning wind tunnel work and a data set that could be used to assess free surface effects.

Most of the remaining 1:3.5 scale tests focused on testing designs for the second canoe body and developing the tandem keel configuration. In May, a canoe was tested which was designed using a wavemaking resistance optimization scheme developed by SAIC. In August, the final two canoe bodies were tested. These designs were developed with input from the Michigan seakeeping tests and SPLASH runs. The final testing completed by the team was an appendage test series completed in February, 1992.

Tank testing was the primary experimental tool

used to develop the tandem keel configuration

since the free surf ace effects were believed to

be critical, and foil position was a major

issue. Three test series were completed,

beginning with a proof of concept series in

February 1991. Based on these results, a more

extensive series of tests was completed in May.

A final test series was completed in August

which was an attempt to better understand the

viscous effects associated with this

configuration.

Table 1 - Summary of 1:3.5 Scale Tank Tests

Date Description

Nov, 1990 Baseline, Displacement,

Stripping

Jan, 1991 Appendage Series

Feb, 1991 Tandem Keel Proof of

Concept

May, 1991 canoe No. 2

May, 1991 Tandem Keel

Aug, 1991 canoes No. 3, 4

Aug, 1991 Large Model Tandem Keel Conventional

TDC Wind Tunnel ~ Program

Table 2 summarizes the tests completed. The

first series of tests was designed to provide

the design team with a set of baseline data and

an improved understanding of how the tunnel

should be used in conjunction with tank tests

and CFO. The model was tested with and without

imaged appendages, and appendages were added

one at a time. In addition, this series

included rudder and flap sweeps and a series of

keel and bulb position variations.

In April of 1991 the first series of bulb and

winglet variations was completed. This was

extended in July and October. Also in October,

a foil test was completed in support of the

tandem keel configuration development. Final

bulb and winglet tests were completed in

January, 1992.

Table 2 - summary of Wind Tunnel Teat Program

Date Description

Feb, 1991 Baseline tests, Keel and bulb position

Rudder and flap sweeps

Apr, 1991 Bulb and bulb/winglet

Jul, 1991 Bulb and bulb/winglet

Oct, 1991 Bulb and bulb/winglet

Oct, 1991 Foil tests

Jan, 1992 Final bulb/winglet

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Structure

Significant effort was devoted to structural

analysis of the first boat by the design team

and consultants from Newport News Shipbuilding

(NNS) and Ove Arup. Structural weight and

stiffness were recognized as major performance

parameters in these new IACC yachts, and the

design team needed to rapidly develop a sound

and efficient structural design for the first

boat.

The structural design of Stars & Stripes, was

complicated by the fact that it was also

intended to be a testing platform capable of

accommodating a wide range of rig and keel

positions. An initial baseline structural

design was developed for FEA modeling and

analysis, which indicated the areas of highest

stress and deflection under the sailing loads

provided by the design team. This FEA model

was then set up for detailing of the model in

the areas of interest and analysis of the

effects of varying the rig and keel positions.

Hull laminates and internal structures were

subsequently modified and optimized to minimize

longitudinal hull deflection and torsional

rotation of the sections between the mast, keel

and rudder. Detailed local models of the mast

step, chainplates and keel attachment structure

were also created to optimize the laminates for

minimum weight with the specified allowable

stress and deflection limits.

Newport News Shipbuilding and the design team

also developed a keel structural analysis

optimization program based on righting moment

maximization at specified heel angles. Design

trade-offs between keel fin size, weight and

bending stiffness were aided by these analyses.

Final keel fin structural optimization was

guided by a detailed FEA model analysis by NNS

of the specified fin geometry developed by the

design team from CFO analyses provided by

Boeing.

FIG. 4 FEA MODEL SHOWING EXAGGERATED DEFLECTION

Design Objectives. In all of the structural design and analysis performed for Stars & Stripes, there were a number of major design objectives that drove the work:

Stiffness The hull and keel were designed to minimize deflections(maximize stiffness) and righting moment.

Adaptability Since the first boat was meant to be a trial horse and test platform, a large amount of adjustability was built into the design.

IACC Rule Laminates were designed to meet the rule for minimum weights and thicknesses, while maintaining maximum stiffness.

Strength A minimum factor of safety was kept within the major structure to avoid costly, time consuming or catastrophic breakdowns.

Analytical Approach. The first step in the structural design was to create an initial, conceptual configuration and use this as a starting point to analyze the impact of reducing material (and weight) or using different structural configurations versus the changes in stiffness. once these initial trade-offs were completed, the preliminary and detailed design stages were initiated. At thie point, the major bulkhead locations were generally fixed, and the design and analysis centered more on the laminates that would meet the minimum rule requirements while providing the maximum stiffness. This led to many time­consuming analysis iterations. The initial keel design studies were also started, with a large effort centered on identifying the best keel fin material based on the trade-off between fin weight, bulb deflection, and cost.

One of the most complicated areas of the detailed design phase was devising a method of attaching the keel fin to the hull to minimize bulb deflection, while allowing a large amount of fore and aft position adjustment. other areas completed during the detailed design

phase include the keel fin structure, bonding requirements for all structural panels and final attachment methods for the keel mast and rigging.

All of these stages used finite element analysis (FEA) to evaluate the designs as they were drawn, leading to a large measure of interaction between the designers and the analysts. The basic approach of the FEA was to use a series of global and local analyses, where design changes were assessed for their impact on both the overall hull stiffness and on local stiffness, such as keel or shroud deflection. The global model consisted of the entire hull and major internal structure, with loads applied to model the keel and rigging.

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This model had a coarser mesh than the local model, but could predict overall hull deflections. The local model was more detailed, with a finer mesh representation of the center section of the boat, spanning the mast step/chainplate area to the rudder. This model predicted the local deflections and gave more accurate prediction of the laminate strengths in these areas.

Once the boat had been constructed and sailed and it became apparent that a second boat would not be built, there were many modifications proposed to reduce the weight penalties that had been built in for keel and mast adjustment and to remove structure in other areas to reduce the pitching gyradius. All of these proposed changes were first modeled and evaluated using the FEA tools before being implemented.

Testing. A material testing program was .undertaken to provide the team members with refined values for the material properties that were used in the structural design and analysis. This project was a joint and cooperative effort between the design team, Goetz Marine Technology, General Motors and the Florida Institute of Technology. In addition to the standard material properties testing,

this test program also covered such areas as beam and bulkhead attachment schemes, where a number of candidates were constructed and tested to evaluate strength and stiffness.

Testing was also performed on the boat to confirm the results of the analytical models. During dockside testing, the rigging and mast were loaded up and hull deflections measured. This and similar tests helped to validate the global FEA model which had previously been used to predict the various deflections. The predicted results gave good agreement with the measurements, allowing somewhat more aggressive design margins to be used.

Strain gauge testing was also performed on the internal hull structure and keel fin. These results validated the local hull and keel FEA models and also provided information about the expected dynamic effects of sailing on keel stresses, so that keel design could be refined for the next iteration.

Conclusions. Based on the initial objectives, the design was very successful. The boat was quite stiff, keel and mast movement were easily accomplished overnight, and there were few structural failures. As could be expected however, Stars & Stripes proved to be somewhat over built for her final use as a competitive IACC yacht. This helped to prevent structural failures, but the weight penalties involved in making her a test platform had to be carried through the racing program.

RESEARCH AND DEVELOPMENT FOR THE FINAL DESIGN

Yf.f Feedback l.!;:Q!!l Stars ~ stripes

The first hull was not tank tested until after

the design was frozen and under construction.

This may, on the surface, sound backwards but

the truth is, the first boat was to be the

baseline for any subsequent development. It

was necessary to know the performance of the

first hull and rig very accurately, and know

how it performed relative to the competition.

As discussed above, there are still significant

aerodynamic and unsteady effects present in

actual sailing conditions which are not modeled

well in the VPP. By determining the

performance of the full size boat accurately,

and knowing the flat water, steady state tank

characteristics of the hull, it is

theoretically possible to determine some of the

aerodynamic and unsteady characteristics.

This is an arduous task, since the performance

characteristics are typically masked by very

noisy data, with a tremendous number of

unmeasured variables, but significant progress

was made in the improved determination of these

variables.

This aerodynamic knowledge can, for example,

now be better modeled in the VPP, which affects

the optimization of the subsequent hull

characteristics and cause optimization to a

different point in the design space.

Seakeeping Experimental Research

Sailing yacht performance prediction, whether

by computer modeling or physical model testing

in a towing tank, is normally examined in a

smooth-water environment. This is because both

means of analysis involve a steady-state

condition of the yacht that is far simpler than

in rough water, where the dynamic interaction

between.the moving sea surface and the moving

yacht introduces additional dimensions of

complexity.

The dynamic response of the yacht to the

encountered waves involves variations due to:

a. hull size, proportions, and details of

shape b. displacement and corresponding

distribution of displacement

c. boat speed d. wave length and height e. yacht's angle of incidence to the waves

f. heel angle g. change in aerodynamics of the pitching

sail plan

All but the last were researched and

satisfactorily quantified by the TDC design

team, with decisive input into the final

design. In fact, earlier knowledge of these

influential factors led to the design team's

choice of parameters in the first-generation

design, Stars & stripes, committed in

September, 1990.

215

As noted previously, PACT had initiated

research in seakeeping codes, which the TDC

design team endeavored to utilize as

appropriate. Meanwhile PACT also sponsored

initial tank test procedures and testing of its

baseline design at the University of Michigan

during February-May, 1991. A limited but well­

chosen model test series was undertaken by TDC

and completed in June 1991. The test program

and investigation of other yacht seakeeping

research focused on the effects of:

a. beam, length, displacement and gyradius

b. wave length, steepness, and incidence

angle c. boat speed

The object.ive was to determine an empirical

prediction scheme for added re~istance of

candidate IACC designs using generalized

relationships of test data to hull

characteristics.

Rough water tank testing is far more time

consuming and costly than smooth water tests.

smooth water is easily defined and created

repeatedly, while waves are not as well

behaved. Target wave profiles are chosen, but

imperfect wave generation and the fact that the

waves change shape as they progress down the

tank add larger error bars to the measurements

than desirable. Measurement of the wave

profile in front of or very near the moving

model is difficult and introduces other

experimental errors.

It should be noted that the TDC testing, and in

fact most seakeeping testing, was done in

"regular" waves. This means waves of constant,

repeating wave length and height as opposed to

the ocean where the sea state is

characteristically called "random". The energy

content of a sea state can be measured and

represented statistically as a function of

mixed component lengths (with a unique

relationship between wave length and

frequency), called a "sea spectrum". Different

wind velocities and other factors cause

different spectra. The energy content of waves

within a limited bandwidth of wavelength can be

integrated from the area under the spectral

curve. Similarly, the entire spectrum can be

defined by values for a finite number of

regular wave lengths (frequencies). See Fig 5.

600

500

N' ( 400

~ = N 300 (

~ "' a:

200

100

~

\

x J J

\

_/

(r-.....

I I\ \

~

"' r--.....

3.0

2.5 u

2.0 r N (

5.

1.5 ~ c: Gl Cl

1.0 ca

i 0.5 CJ)

r--0 0.0

FIG 5.

0 25 50 75 100 125 150 175 200 Wave Length (fl)

SEA SPECTRUM AND ADDED RESISTANCE OPERATOR

Because the random sea state itself can be described by the superposition of regular waves, it follows that models can be tested in regular waves and the results then extrapolated to any arbitrary sea state by spectral analysis. In fact, testing in approximately 10 regular wave lengths adequately describes the response of the yacht's motions and added resistance for making random sea predictions. (See added resistance operator on Fig. 5). Another convenient mathematical relationship that appears to hold true enough for yachts is the linearity of responses to wave height. In the case of motions, doubling the wave height produces twice the motion amplitude and added resistance increases approximately in proportion to wave height squared. By multiplying the resistance "response amplitude operator" for added resistance by the "sea spectral density", one obtains the added resistance curve (Fig. 6), whose area (when consistent units are used) is the added resistance of the yacht in the prescribed conditions.

Because only the total resistance of the model can be measured in the towing tank, added resistance is the total resistance minus the calm water resistance. There are enough physical variables in rough water testing to require several runs to be made at each boat speed and wave length and height to determine a statistically significant value of resistance. It can take half an hour or more for the water to be calm enough for the next run to be made, and an accurate set of calm water data for the model must be taken as well. Given TDC's priorities, limited time and money, analysis concentrated on added resistance, rather than motions, although they also were measured and logged.

By testing a systematic series of models, the quantitative effects on added resistance due to a change of each of several key parameters could be measured. After considerable effort in data analysis, relationships were found between the principal characteristics of the models and added resistance so that nearly all of the test measurements could be predicted within experimental accuracy. With the confidence of this generalized, rough water predictive ability, the design team could make accurate allowances for added resistance in waves in drag calculations of candidate designs for analysis by the VPP.

YS

450

400

~ 350 g. 300 E 2 250

i en 200 ., ~ 150 0

~ 100 er:

50

0 0

I\ I I~ \ I \

' \.....,

I \

I ""' u -....... ---25 50 75 100 125 150 175 200

Wave Length (ft)

FIG 6. ADDED RESISTANCE CURVE

216

Seakeeping turned out to be a very important factor in the outcome of the 1992 America's Cup, as anticipated by the design team. Considering the size of the matrix of test conditions and test cycle times, one can appreciate that seakeeping tests are rarely undertaken in yachts--even for the America's cup. More progress in both testing and analytical solutions is recommended for the next America's Cup.

Volume Distribution

The design team worked together with the Ship Hydrodynamics division of SAIC LaJolla to investigate the effects of hull volume distribution on wave and total resistance. Initially, optimized upright sectional area curves were developed with a thin-ship based wave drag code, which included estimated viscous drag effects, for IACC hull forms at lengths, volumes, buoyancy centers and target speeds as specified by the design team. A comparison of these results with the TDC baseline hull form provided direction for the development of an alternative hull form for testing.

In an effort to further investigate and optimize the details of the hull form, a slender-ship based hull geometry optimization code developed by SAIC LaJolla was employed to generate optimum hull forms within the naval architectural constraints specified by the design team. This panel-method code, which combined the slender-ship theory wave resistance calculations and ITTC (with form correction) viscous drag estimates to predict total resistance, was modified to optimize heeled, as well as upright, hull forms at the speeds specified by the design team.

Initial studies with the code generated some irregular hull forms which included bumps and hollows precluded by the IACC rule. The code, which iteratively moves panels on the hull surface as it converges towards the minimum resistance configuration, possessed the useful capability of producing gradient plots which indicated which panels could be moved with the least impact on the total resistance. With the gradient plots providing design guidance, a smooth hull form was created which the code predicted to have significantly less total resistance at speeds above 8.2 knots and similar or slightly greater resistance at lower speeds.

While the above results appeared promising, questions remained regarding the reliability of the method and the predicted form drag associated with the resultant hull shape. It was determined that a towing tank model test was needed to establish the true potential of the optimized hull form.

The results of the tank test at Arctec were

less encouraging, with the total resistance

being similar to or greater than the TDC

baseline in all conditions, although the hull

form factor was determined from Prohaska plots

to be similar to the baseline. An analysis of

the longitudinal wave cut data using Sharma's

method for calculating wave drag failed to

conclusively reveal any increase in wave drag

for the model relative to the baseline,

However this now appears to have been due to a

mis-calibration of the wave drag probes. It

was noted that the measured sinkage and trim of

the optimized model were both greater than the

baseline, which was attributed to changes in

the afterbody shape. Based on these results,

the design team elected to return to the use of

thin-ship based optimal sectio~al area curves

for guidance on hull volume distribution.

Structural Optimization

Design Objectives. The design objectives

for the second boat were similar but not

identical to the first boat:

Stiffness

Weight

The hull and keel were designed to

minimize deflections(maximize stiffness)

and righting moment.

A major effort was made to reduce

unnecessary weight, especially by fixing

the keel and mast positions, integrating

major structural components, and removing

redundant structure.

IACC Rule Laminates were designed to meet the rule

for minimum weights and thicknesses,

while maintaining maximum stiffness.

Strength A minimum factor of safety was kept

within the major structure to avoid

costly, time consuming or catastrophic

breakdowns.

Analytical Approach. For this design,

the team already had a strong baseline to work

with from the design iterations performed for

the first boat. Therefore, the basic approach

was to take what was learned from testing and

sailing the first boat and from observing the

competition and improve upon that. There were

three major paths taken in the structural

optimization of the second boat:

a. Eliminate internal structure (bulkheads,

longitudinals, etc.) or relocate for

maximum stiffness and minimum weight.

b. Optimize ply orientations of all major

laminates in the hull structure for

stiffness.

217

c. Reconfigure the deck to lower the hull

center of gravity, reduce hull weight,

and increase stiffness.

General Motors performed a design of

experiments on the global FEA model in which

the variables were either stringer location or

removal. Many combinations of variables were

analyzed, and the results graphed to find the

greatest structural benefit dependent on these

variables. In this case, the objective was to

reduce overall hull deflection (increase hull

stiffness). These analyses were used to

streamline the internal structural

configuration and greatly reduce the weight,

while maintaining desired hull stiffness.

The laminate optimization was also performed by

General Motors. Here performing a design of

experiments would have been prohibitively

costly and time consuming due to the nearly

infinite number of combinations of ply

orientations in the hull and hull structure.

Instead the internal design optimization

capabilities of NASTRAN were used. In NASTRAN

Version 67, the program uses a directed

optimization routine to evaluate changes in

structural properties such as plate thickness

or laminate ply orientations, and finds the

best combination of these properties based on a

given objective function. The objective here

was to optimize hull stiffness while keeping

the laminate at the IACC Rule minimums.

The hull and internal structure were broken up

into about 20 separate laminates, and each of

the ply orientations within was allowed to

vary. The objective function (minimum bending

and torsional deflection) was input, and

NASTRAN calculated the resulting optimized

laminates. These laminates were then modified

slightly to make them practical for

construction.

The deck configuration studies were conducted

jointly by the designers and Newport News

Shipbuilding. Several proposed configurations

were chosen by examining the changes made by

other teams and looking at our own design

objectives and how they could be achieved.

These configurations were then input into the

global FEA model and evaluated based on overall

hull stiffness, torsional stiffness in the

cockpit area, hull weight, and hull center of

gravity.

Results. Since the second boat was never

built, it is difficult to tell for certain the

gains made over Stars & Stripes(USA 11), or how

well it would have fared in competition. The

analytical results, however indicated that

between the improvements found through each of

the three optimization paths, the second boat

design was approximately 15% stiffer than the

first, and had a considerable (but still

confidential) improvement in weight and center

of gravity.

VPP/Parametric Optimization

A significant effort was expended, particularly with the new class of yacht to be designed, to determine the tradeof fs within the IACC Rule for the basic parameters of length, sail area, and displacement. In addition, the waterline and deck beam must be traded off against the necessary stability. While the beam doesn't appear in the class rule, it is a very important parameter.

For example, since the basic weight of the hull and rig has a practical minimum due to the skin scantlings and mast weight requirements, there is a limited amount of stability available due to displacement or the position of the vertical center of gravity (VCG). Increasing the beam increases the boat's stiffness, but at the expense of added drag, particularly in ocean waves. Similarly, increasing the displacement increases the stiffness at the expense of higher drag, but within the rule, adding displacement buys more sail area.

As every designer knows, adding length will increase the boat's speed potential, at least above drifting conditions, but the IACC rule reduces sail area if the length is increased. There is also the issue of the bow shape and its effect on the measured length. The long, overhanging bow shape has a shorter static waterline length than the destroyer type bow, for the same rated length, but tends to pick up sailing waterline length with speed and heel, which the destroyer bow does not.

A method to find the optimum set of parameters for the expected conditions of seastate and wind was required. As often the case with real-life problems, the process requires optimization of several variables without enough data input. A combination of LPP derivatives and statistical regression was used to find the "sweet zone" in the design space. These methods were applied to the design of the "second boat" which was tank tested and predicted to be a dramatic step forward from the baseline Stars & Stripes, but which was never built.

APPENDAGE RESEARCH AND DEVELOPMENT

Basic Appendage Considerations

IACC racing yacht appendages need to perform two primary functions:

a.

b.

Rigidly support ballast as required for lateral roll stability

Generate lateral forces required for tracking and maneuvering

These requirements must be met while producing a minimum amount of drag over a wide range of speeds, heel angles and loads. The maximum draft of the yacht in measurement trim is effectively limited to 4.00 meters by the IACC rule which also limits the span of any winglets and the number of moveable appendages and axis position of any moveable appendages to within the centerplane of the yacht.

218

Ir. order to analyze and ration&lly develop an optimized appendage configuration for an IACC yacht, it was first necessary to define the operational conditions and quantify the performance deltas associated with changes in drag and stability over the range of operating conditions. VPP derivative studies were used to derive these deltas, including the effects of changes in stability due to lateral keel bending. Drag polars produced for each configuration were then integrated over operational values for lift to predict performance deltas for configurations with sim.ilar stability characteristics. The deltas could be adjusted for the effects of differing stability from the VPP derivative studies. Weighting factors based on statistical wind speed data were then applied to the performance deltas to develop a composite overall performance figure of merit for each configuration.

Monoplane Configuration

The conventional, or monoplane, keel configuration was approached with the above criteria in mind, starting with an analysis of the hydrodynamic loading conditions that must be sustained. The VPP and other proprietary keel/rudder trade-off codes were employed to determine a basic lateral area for the keel and rudder, knowing the tank test trim tab, leeway and load sharing optima. This determined a starting point from which the structural analysis could be undertaken.

The goal of the structural tradeoff is to maximize the righting moment with some trade­of f for drag. For example, if the keel were made thicker to minimize the bending, (bending results in a loss of righting moment) the increased thickness would add drag which must be tolerated all around the course.

The selection of the keel sectional shape had to consider the structural aspect as well as the hydrodynamic effects. Some sectional shapes provide more stiffness but not necessarily less drag or less weight. The trade-offs here relate to the position along the chord line of the maximum thickness, therefore the structural stiffness, but also the keel's drag characteristics in laminar or turbulent flow as well as its tolerance to transient higher load conditions, such as tacking.

The selection of materials for the keel is another righting moment trade-off, with cost as a non-trivial parameter. A composite keel for example, is lighter than its steel counterpart, but for the same sectional shape, will deflect more under load, and therefore the net righting moment must be optimized differently in terms of structure and section, for different materials.

Later generation keels tended to get smaller, as the range of loadings became better known and the upwind/downwind trade-offs became better understood on the race course. In addition, winglets sprouted in the later stages, which change the keel optimization.

Ballast Bulbs. The function of the ballast

bulb is conceptually very simple,. It provides

most of the righting moment required to sail

upwind. As such, it is generally desirable to

have the maximum weight of highest density

material, in the lowest drag arrangement

possible.

The IACC rule limits the bulb material such

that nothing denser than lead can be used, and

limits the maximum draft of the keel/bulb to

4.0 meters. A round torpedo-like bulb was the

starting point since it has reasonable drag for

the necessary volume. However it was soon

obvious that squashing the bulb somewhat lowers

the vertical center of gravity as well as

allowing a longer keel span (since the bulb on

the end of the keel robs the keel of aspect

ratio and thus increases its induced drag).

Squashing the bulb, however, isn't magic,

since the surf ace area of the bulb increases

rapidly and therefore the drag of the bulb

itself increases.

A significant effort went into the TDC bulb

design program in an effort to find the best

compromise of shape for the bulb. The optimum

shape considers all of the above, attempting to

pack the lead volume low (for stability

reasons), trying to minimize the adverse span

effects, and of course ensuring that it does

not fall off under the worst possible loading

case.

Winglets. Winglets have been around

aerodynamic circles for many years, and made a

well known appearance in the 1983 America's Cup

on Australia II. Their theoretical benefits

and trade-offs are very complex but can be

summarized as follows:

In the IACC class, the draft is limited to 4.0

meters which limits the keel span to something

less than optimum. By adding carefully

positioned winglets attached to the bulb,

sloping downward lightly, the induced drag can

be reduced, resulting in a slight improvement

upwind. Unfortunately, adding this surface

area below the water results in higher viscous

drag. For the winglets to be a net gain, the

induced drag benefit upwind and tacking (and to

a much lesser extent, reaching) must outweigh

the added viscous drag which must be carried

upwind and downwind. .

In summary, winglets can be made to improve the

IACC yacht performance upwind at some penalty

downwind. The design trade-off study revolves

around this upwind/downwind strategy and the

optimization of size, shape and position of the

winglets.

Biplane Configuration

The ~ ~ Stripes Tandem. During the

Challenger Series for the 1987 America's cup,

the "twin rudder" USA demonstrated that there

were high performance alternatives to the fin

keel monoplane. Therefore, it was necessary to

study multiplane designs in preparation for the

1992 America's Cup.

219

USA, however, was developed under the 12 Meter

Rule, in which a short draft keel is coupled to

a deep draft canoe body. The 12 Meter Rule led

to the structural optimization of USA with tri­

plane appendages. This enabled the complete

separation between the ballast function

(assigned to a lead torpedo supported by a

short ventral strut) from the hydrodynamic

sideforce function carried out by the two

foils, (fore and aft of the ballast). Although

geometrically a triplane, the "twin rudder" USA

was, from a hydrodynamic standpoint, a high

performance biplane. Due to the significantly

different displacement and draft

characteristics of the new IACC rule, as

opposed to the 12 Meter Rule, it was necessary

to begin the 1992 multiplane design effort with

a clean sheet of paper.

Various multiplane appendage solutions were

considered within the constraints of the IACC

Rule, attempting to match the Stars & Stripes

canoe body, which had originally been designed

as a conventional monoplane. No attempt was

made to design an integrated canoe/multiplane

solution.

The TDC multiplane appendage work was carried

out over a 13 month period from the

commencement of R & D until the design freeze

for full size fabrication. A brief review of

the TDC tandem program is presented below.

Benefits of Multiplanes. The general

multiplane configurations which are attractive

under the IACC Rule considered by the TDC

design team were:

a. USA type triplane (Fig. 7)

b. twin foil "tandem" approach

The triplane solution, as examined by the

design team, encountered adverse wake

interference problems when constrained by the

IACC Rule limitation of two movable appendage

surfaces. It also required very critical

volume/wetted area/span tradeoffs, which had to

be evaluated with hydroelastic considerations.

Given limited resources, and time constraints,

this approach was not pursued beyond initial

feasibility studies.

On the other hand, the twin foil tandem

solution appeared less restricted by the IACC

Rule, and offered the promise of several

significant benefits compared to the classical

monoplane fin keel configuration. Two of the

tandem benefits are amenable to brief

discussion within the scope of this paper.

FIG 7. TRIPLANE

Tandem Induced Drag Benefits. According to wing theory, if the maximum span of a wing system is specified, minimum induced drag is obtained by an equal-spanned multiplane of maximum gap. The relation between induced drags of a monoplane, Dim, and of a biplane wing Dib having a wide biplane gap Gb between its wing members is discussed in Ref. 4, summarized for convenience in the following equation:

and for the biplane:

Dib={(L1/b1) 2+2*z*L1*L2/b1*b2+(L2/b2)2}* (l/Pi*q)

( 1)

(2)

Equation 2 was chosen for elliptic loading to clearly show the distribution of the induced drag forces of the wing members of the biplane, with the middle term quantifying the mutual induction effects.

If the spans and lifts of the wings are made equal, governed by a draft limit, for example, it can be shown that the induced drag efficiency ratio of the biplane relative to the monoplane is:

(3)

According to equation (3), for equal spans, the biplane's induced drag with values of z less than 1 is always less than the monoplane and this occurs when the gap between the wings of the biplane has finite values. For zero gap, z becomes 1, and the biplane's induced drag advantages are lost.

The classical aircraft configuration for equation (3) is the unstaggered biplane with large gap. It is noted, however, that the unstaggered feature serves only a structural purpose (i.e. efficient wire bracing), since, according to equation (2), induced drag is independent of stagger. (Stagger is the distance between front and rear members of a biplane wing in the chordwise direction.)

The classical gapped biplane airplane configuration cannot be applied to an IACC Rule yacht because all appendages must be on the hull centerline. This does ~ot rule out a centerplane biplane as an appendage configuration utilizing a staggered twin foil keel properly called a "tandem" (Fig. 8). This design approach, however, would appear to rule out the induced drag benefit of the biplane configuration shown in equation (2), since by definition, a centerplane tandem must have zero geometric gap between its foils.

Nevertheless, equation (2), under upwind sailing conditions, can be properly interpreted with certain care, as having a significant hydrodynamic gap, even if the foil's centerplane geometric gap is zero.

220

Equation (3) for a yacht must be handled with care since its formulation in wing theory assumes a limitless flow field. Therefore, its application to a yacht requires the mathematical substitution of half of the limitless flow field on one of the aerodynamic biplane's sides, (say at a constant geometry boundary of y=O) by the yacht's water-air interface applied adjacent to the roots of the yacht's tandem foils (a constant pressure boundary which is not, however, a gravitational equipotential in the vicinity of the hull). Thus, equation (3) is not rigorously applicable to a yacht. The theoretical treatment of the necessary water-air interface substitution is clearly beyond the scope of this paper.

It should also be remarked that equation (3) cannot compare a biplane against a monoplane having winglets, unless equation (1) is changed first, increasing the monoplane's effective span by the positive effect of the winglet.

FIG. 8 TANDEM

Tandem Structural Benefits. The rationale for a tandem within the IACC Rule may be derived from considerations other than induced drag benefits under multiplane wing theory. This is because the principle IACC monoplane solution is a high aspect ratio fin keel with a streamlined bulb trailing the fin's

tip, as in Fig. 9. This type of bulb design offers low interference drag with the fin keel, and also minimizes kelp entanglement since the bulb does not protrude forward of the fin at the intersection. However, a trailing bulb has certain adverse structural characteristics which develop as the boat heels:

1. The weight of the trailing bulb twists the slender fin keel, reducing the righting moment contributed by the bulb's center of gravity.

2. The ensuing torsional deflection of the slender fin keel opens up the potential for hydroelastic flutter, at least at the high speeds encountered when reaching (water dynamic pressures of approximately 400 lb/ft2)

Both of these problems can be greatly

alleviated if the conventional rudder of Fig. 9

is strengthened and moved forward on the canoe,

as shown, with the rudder's span extended

downward and forward to structurally engage and

support the rear end of the bulb. This type of

structural solution greatly decreases the

flutter potential and reduces the decay of

righting moments with heel which otherwise

characterize the trailing bulb/keel

configuration of Fig. 9. Nonetheless, the

tandem solution requires careful and rigorous

trade-off studies between foil stiffness and

wetted area.

!

ow~~-

u JJ FIG. 9 MODIFIED MONOPLANE

Tank Test Results. The TDC tandem

configuration was first developed

theoretically. It was then tank tested using

small scale models at Reynolds numbers known to

be very low. Results were sufficiently

favorable to justify a short but successful

program at 1/3.5 scale at the Arctec towing

tank. These tests confirmed a significant

upwind benefit, compared with the TDC baseline

configuration. The gains in lift/drag ratio

upwind were of sufficient magnitude to warrant

moving forward with a tandem program for manned

sailing on a half sized boat.

Hobie 33 Tandem Keel Tests. The TDC

design team realized early that sailing a

tandem appendage configuration new to the

America's Cup required a significant on the

water time to develop maneuvering techniques

and to optimize sails for the configuration.

Thus the original tandem program at TDC was to

include 1/2 scale IACC yachts, one tandem and

the other a baseline monoplane configuration to

investigate handling qualities and performance.

This was a cost effective and scientifically

valid procedure to optimize the design of the

full size boat through sailing tests of it

smaller counterpart, prior to fabrication of

full size appendages. It also would have

reduced the adverse impact of not having a full

size trial horse in the TDC program.

Unfortunately, due to funding limitations, the

exact one half scale boats were not built.

Instead, an attempt was made to evaluate the

sailing qualities of the tandem using

lengthened Hobie 33's. They reasonably closely

approximated 1/2 scale displacement, length,

and beam but in order to scale the righting

moment, the bulb densities had to be altered.

221

The other non-scale element was the sail area

and rig height, partially compensated for with

new fully battened, large roach mains.

Performance comparisons were difficult since

the non-scaled nature of the boats made

mathematical extrapolation to a full size IACC

yacht impossible but the handling qualities of

the tandem were found to be satisfactory, and

the decision was made to proceed with full size

appendage fabrication for retrofit to Stars &

stripes.

Sailing the Stars ~ Stripes Tandem. To

compensate for the lack of accurate one half

scale sailing tests, TDC endeavored to get as

much sailing time for the full size tandem as

possible before the start of racing. An

experimental full size optimization of

sail/foil yaw balance was practical due to the

large range of mast position possible in the

deck and structure of Stars & Stripes

(fortuitously designed with a large range of

movement since this was to be a test boat

anyway). Tests and sailing trials were planned

for the month preceding the first Defender

Round Robin in January, 1991. The timing and

strategy were planned to allow the use of the

opponent as the test boat in the first, low

scoring Defender round.

Delays in design finalization, and fabrication

of the tandem appendages, however, forced Stars

& Stripes to race the first round robin with

its proven and efficient monoplane

configuration. These races confirmed that

Stars & Stripes had good performance level

compared to Jayhawk, but was less successful

against Defiant.

TDC's tandem appendages were installed in stars

& Stripes after the first round robin, a

Herculean task including removal of the

monoplane appendages and controls, installation

of new structure in the canoe, installation and

alignment of the tandem appendages and totally

new control system.

Thus the Stars & Stripes tandem configuration

entered round robin 2 under less than ideal

conditions and preparation. There were only

four days in the water prior to the start of

round 2 to carry out the IACC required rule

flotation measurements, check and debug the

control system, and tune the boat (primarily

yaw balance) with no trial horse and whatever

limited wind conditions were present. In

addition, the surface finish of the appendages

and joint fairings were incomplete due to time

limitations.

It is a matter of record that the Stars &

Stripes tandem lost the only 3 races it sailed

by large time margins. A significant yaw

balance problem became evident in the strong

winds of races 2 and 3 forcing depowering of

the main when close-hauled, and was obviously

damaging to the upwind performance.

The large time losses of the tandem in the first 3 races of round robin 2 compared to the performance of the monoplane in round robin 1 were attributed by most observers as a configuration problem inherent in the Stars & Stripes tandem.

However, the assessment of the tandem's performance in round robin 2 was difficult to make for two reasons. One was heavy seas, which had not been present during the first round robin. As it turned out, the largest waves of the entire Defender trials were encountered by the Stars & stripes tandem in round robin 2. The second factor was the speed of the new America3 boat USA 23, the eventual winner of the America's Cup making its first appearance in round robin 2.

A closer look at some of the performance highlights of the tandem sheds a somewhat different light on the configuration, however. For example:

February 8 In relatively calm water and light winds (which make the yaw imbalance less critical), the Stars & Stripes tandem showed its most promising upwind performance against America3 , particularly in the lighter, flatter early stages.

February 11 In conditions unusually rough even for San Diego, while suffering upwind due to yaw balance problems, Stars & Stripes was able to gain a total of 23 seconds against America3 on the downwind legs.

Immediately after the February 11 race, the America3 syndicate offered TDC the option of withdrawing the tandem configuration and replacing it with the original monoplane, but declined to allow the changing of the mast position, which would have helped alleviate the yaw balance problem with the tandem. Thus, to avoid further large losses, while still trying to raise funds to carry the syndicate through to the America's Cup, TDC had no alternative but to replace the tandem with the monoplane configuration and continue with round robin 2. Winning the first race with the re-fitted monoplane provided a much needed morale boost to the sailing team.

Lack of time to sail and tune or refine the tandem configuration before entering the Trial races undoubtedly contributed to its disappointing debut, and perhaps masked the true potential of the configuration which was thus never realized.

222

CONCLUSIONS

The Stars & Stripes design effort was a cost effective, multi-disciplined program which both utilized available technology and advanced the state of the art in many areas. It boldly and briefly experimented with a retrofit tandem configuration, the potential of which was unreached in the TDC one boat campaign with limited resources. With no option but to concentrate its sailing and development efforts on its only boat, the team was successful in extracting the utmost potential from the original design.

REFERENCES

1. Beck, R., et al, (1989) "Motion in Waves," SNAME Principles in Naval Architecture, Vol. III, Chapter 8.

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3.

4.

Gerritsma, J., "Performance of Light and Heavy Displacement Sailing Yachts in Waves," Second Tampa Bay Symposium SNAME, Sec. 1, February 1988.

Pedrick, D., "The Performance of Sailing Yachts in Oblique Seas," Chesapeake Symposium SNAME, No. 10, January 1974.

Calderon, A.A., "The Keelless Twin Wing 12-Meter Yacht USA", Volume 34, Ancient Interface XVII, 1987.